1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copier, a printer, or a facsimile machine, that includes two or more latent-image carriers of which the surfaces go around the respective latent-image carriers, such as photoconductors, to be aligned in a surface moving direction of an object onto which an image is to be transferred, such as an intermediate transfer medium or a recording material, and obtains a final image by transferring visible images, which are obtained by developing respective latent images on the surfaces of the latent-image carriers, onto the object in a superimposed manner.
2. Description of the Related Art
As this type of image forming apparatus, for example, a tandem-type image forming apparatus including four photoconductors (latent-image carriers) for forming yellow, magenta, cyan, and black visible images, respectively, has been conventionally known. In the image forming apparatus that transfers respective visible images formed on the photoconductors onto an object in a superimposed manner, to reduce a degree of relative transfer misalignment among the visible images transferred onto the object (hereinafter, arbitrarily referred to as “color shift”) is important in improving the image quality.
As causes of color shift, there is radial run-out of a photoconductor driving gear (a driven transmission rotating body), which is fixed to a rotating shaft of a photoconductor, due to eccentricity of the photoconductor driving gear. The color shift caused by the radial run-out is explained in detail below. The photoconductor driving gear has the radial run-out due to its own eccentricity, and rotates at the lowest angular velocity when a portion of which having the longest radius engages with a motor gear or an idler gear that transmits a rotational driving force to the photoconductor driving gear. Thus, if other fluctuation components that can fluctuate the linear velocity of the photoconductor are not taken into consideration, the photoconductor provided with the photoconductor driving gear has the lowest angular velocity at this time, and also has the lowest linear velocity at this time. Furthermore, from the same point of view, when a portion of the photoconductor driving gear having the shortest radius engages with the motor gear or the idler gear, the photoconductor driving gear rotates at the highest angular velocity, and the photoconductor provided with this photoconductor driving gear has the highest linear velocity. The former portion causing the photoconductor to have the lowest linear velocity and the latter portion causing the photoconductor to have the highest linear velocity are located at positions symmetrical with respect to a point of the rotation center of the photoconductor driving gear, i.e., at rotational positions different by 180°. Therefore, the angular velocity of the photoconductor driving gear has a sinusoidal fluctuation component with a period corresponding to one revolution of the photoconductor driving gear, and thus the sinusoidal fluctuation component with the period corresponding to one revolution of the photoconductor driving gear is seen in the linear velocity of the photoconductor. A toner image (a visible image) transferred onto the object from the photoconductor when the photoconductor rotates at the linear velocity of around the upper limit of the fluctuation component has a contracted shape that an original shape is contracted in a sub-scanning direction (the surface moving direction of the object). In contrast, a toner image transferred onto the object from the photoconductor when the photoconductor rotates at the linear velocity of around the lower limit of the fluctuation component has an elongated shape that an original shape is elongated in the sub-scanning direction. Accordingly, when a toner image on one of two photoconductors and a toner image on the other photoconductor are transferred onto the same point on the object, if one of the toner images has the most contracted shape and the other toner image has the most elongated shape, the maximum degree of color shift occurs.
Usually, the same gears are used as the photoconductor driving gears provided to the photoconductors, so it can be said that an amplitude value of radial run-out of each of the photoconductor driving gears due to its own eccentricity is the same. Therefore, an amplitude value of the fluctuation component seen in the linear velocity of the photoconductor due to the eccentricity is the same, and a maximum amount of elongation/contraction of a toner image transferred onto the object due to this is the same. Therefore, if relative rotational positions of the photoconductor driving gears are adjusted so that toner images having the most contracted shape or toner images having the most elongated shape are transferred onto the same point on the object, color shift due to the eccentricities of the photoconductor driving gears can be prevented in theory.
As a configuration for driving three or more photoconductors, there has been conventionally known a configuration that a motor gear (a drive transmission rotating body) connected to a drive source is directly connected to each two photoconductor driving gears of those provided to the photoconductors thereby driving two photoconductors provided with the two photoconductor driving gears. In this configuration, by adjusting a phase of eccentricity of the photoconductor driving gear provided to one of the two photoconductors at a point of time when a specific point on an object (an arbitrary point in the surface moving direction of the object) passes through a transfer section of the one photoconductor to coincide with a phase of eccentricity of the photoconductor driving gear provided to the other photoconductor at a point of time when the specific point passes through a transfer section of the other photoconductor, color shift due to the eccentricities of the photoconductor driving gears can be prevented in theory. However, in this configuration, at least two drive sources are necessary, and problems of rising cost and difficulty in downsizing of the apparatus occur.
On the other hand, as another configuration for driving three or more photoconductors, there has been also known a configuration that a motor gear connected to a drive source is directly connected to some of photoconductor driving gears and is connected to the rest of the photoconductor driving gears via another photoconductor driving gear and an idler gear (a driven rotating body) (see, for example, Japanese Patent Application Laid-open No. 2003-329090 and Japanese Patent Application Laid-open No. 2004-117386). In this configuration, all photoconductors can be driven by the single drive source, and thus it is possible to achieve cost reduction and downsizing of the apparatus as compared with the foregoing configuration that the motor gear is directly connected to the photoconductor driving gears without using an idler gear.
However, the conventional configuration using the idler gear has a problem that even if relative rotational positions of two photoconductor driving gears connected to each other via the idler gear are adjusted as described above, color shift due to the eccentricities of the photoconductor driving gears still occurs.
This problem is explained with an example where the two photoconductor driving gears connected to each other via the idler gear are composed of the photoconductor driving gear directly connected to the motor gear connected to the drive source (hereinafter, referred to as a “second photoconductor driving gear”) and the photoconductor driving gear to which a rotational driving force is transmitted through the idler gear that rotates in accordance with rotation of the second photoconductor driving gear (hereinafter, referred to as a “first photoconductor driving gear”). In this example, eccentricity of the photoconductor driving gear that affects the fluctuation component of the linear velocity of the photoconductor provided to the second photoconductor driving gear (hereinafter, referred to as a “second photoconductor”) is only eccentricity of the second photoconductor driving gear provided to the second photoconductor. On the other hand, eccentricity of the photoconductor driving gear that affects the fluctuation component of the linear velocity of the photoconductor provided to the first photoconductor driving gear (hereinafter, referred to as a “first photoconductor”) includes not only eccentricity of the first photoconductor driving gear provided to the first photoconductor but also the eccentricity of the second photoconductor driving gear transmitted via the idler gear. In other words, the angular velocity of the first photoconductor driving gear includes composite wave of the fluctuation components due to the eccentricities of the both photoconductor driving gears (hereinafter, referred to as a “composite-wave fluctuation component”); as a result, this composite-wave fluctuation component is seen as a linear-velocity fluctuation component in the linear velocity of the first photoconductor.
In this configuration, when the adjustment described above is made, relative rotational positions of the first photoconductor driving gear and the second photoconductor driving gear are set so that a phase of the composite-wave fluctuation component of the angular velocity of the first photoconductor driving gear at a point of time when a specific point on the object passes through a transfer section of the first photoconductor coincides with a phase of the fluctuation component of the angular velocity of the second photoconductor driving gear due to the eccentricity of the second photoconductor driving gear at a point of time when the specific point passes through a transfer section of the second photoconductor. Consequently, toner images having the most contracted shape or toner images having the most elongated shape are transferred onto the same point on the object.
If a distance between the transfer sections of the first and second photoconductors is configured to be equal to an integral multiple of the circumferential length of these photoconductors, even when the same gears are used as the photoconductor driving gears, an amplitude value of the composite-wave fluctuation component of the angular velocity of the first photoconductor driving gear can coincide with an amplitude value of the fluctuation component of the angular velocity of the second photoconductor driving gear due to the eccentricity of the second photoconductor driving gear. Therefore, if this configuration is employed, color shift due to the eccentricities of the photoconductor driving gears can be prevented by the adjustment described above.
However, if this configuration is employed, the internal layout of the image forming apparatus is much limited, and it is not possible to meet demands, for example, a demand to downsize the apparatus as compact as possible by reducing the distance between the transfer sections to be smaller than the integral multiple of the circumferential length of the photoconductors as much as possible. Furthermore, this configuration may not be employed by other limitations. Therefore, in the conventional image forming apparatus, generally, the distance between the transfer sections of the first and second photoconductors is configured to deviate from a value of the integral multiple of the circumferential length of these photoconductors. In this case, an amplitude value of the composite-wave fluctuation component of the angular velocity of the first photoconductor driving gear does not coincide with an amplitude value of the fluctuation component of the angular velocity of the second photoconductor driving gear due to the eccentricity of the second photoconductor driving gear. As a result, an amplitude value of the linear-velocity fluctuation component of the first photoconductor does not coincide with an amplitude value of the linear-velocity fluctuation component of the second photoconductor, and thus an amount of contraction in the sub-scanning direction of a toner image having the most contracted shape on the object or an amount of elongation in the sub-scanning direction of a toner image having the most elongated shape on the object differs between the first photoconductor and the second photoconductor. Therefore, even if it is adjusted so that toner images having the most contracted shape or toner images having the most elongated shape are transferred onto the same point on the object, color shift corresponding to a difference in amount of contraction or elongation (hereinafter, referred to as “specific color shift”) still occurs.
The specific color shift can be prevented from occurring by making the adjustment described above if separate rotating bodies having a different amount of eccentricity from each other are used as the first and second photoconductor driving gears and if an amount of eccentricity of the first photoconductor driving gear is set to an amount capable of eliminating the specific color shift. However, using gears having a different amount of eccentricity from each other as the first and second photoconductor driving gears becomes a factor causing the rising cost, and the difficulty of manufacturing the first photoconductor driving gear having an amount of eccentricity capable of eliminating the specific color shift is another factor causing the rising cost.
The present invention is made in view of the above problems, and an object of the present invention is to provide an image forming apparatus capable of reducing a degree of specific color shift that may occur between two driven transmission rotating bodies, such as photoconductor driving gears, connected to each other via a driven rotating body, such as an idler gear, even if the same rotating bodies are used as these driven transmission rotating bodies when a distance between transfer sections of first and second latent-image carriers is configured to deviate from a value of an integral multiple of the circumferential length of these latent-image carriers for downsizing of the apparatus or the like.